Intermodulation distortion analyzer

Abstract

An intermodulation distortion analyzer generates two pairs of sinusoidal test tones to serve as a test signal for the channel under test. The two pairs of tones simulate two respective noise band test signals but eliminate the long time averaging required for measurements when noise bands are used. A highly linear AGC circuit employs sampling at an output-controlled duty cycle to maintain a constant reference level for the analyzer. This reference level permits automatic distortion measurements to be read out directly in db below the test signal. An RMS detector circuit for second order intermodulation products employs feedback control to maintain the input signal to a squaring circuit constant. Squaring of the constant level sinusoids produces RMS DC components which can be separated for direct measurement. A distortion circuit provides known levels of second and third order intermodulation in the test signal to permit accurate check out of the analyzer.

What is claimed is: 
     
       1. An RMS detector circuit comprising: an input terminal;   an output terminal;   a squaring circuit for receiving a first signal and providing a squared signal having an amplitude which is proportional to the square of the amplitude of said first signal;   gain control means for automatically adjusting signals applied to said input signal terminal to a signal at a substantially constant level, said gain control means including feedback means for maintaining said substantially constant level an inverse function of signal amplitude at said output signal terminal;   means for applying said signal at substantially constant level to said squaring circuit as said first signal; and   low pass filter means for eliminating all but DC components in the squared signal from said squaring circuit and applying said DC components to said output signal terminal;   wherein said gain control means comprises:   a high gain operational amplifier having an input terminal and an output terminal which provides an inverted version of the signal at said input terminal;   an operational transconductance amplifier having an input signal terminal, a bias control terminal, and an output terminal which provides an output current proportional to the product of the voltage at said input signal terminal times the current delivered to said bias control terminal;   means connecting the output signal from the output terminal of said operational amplifier to the input signal terminal of said operational transconductance amplifier;   a voltage to current converter for providing a control current proportional to the voltage at the output terminal of said RMS detector;   means for applying said control current to said bias control terminal; and   means for applying the current delivered from the output terminal of said operational transconductance amplifier to the input terminal of said operational amplifier.   
     
     
       2. The distortion analyzer according to claim 1 wherein said communication channel under test is a telephone line and said known passband is in the audio frequency range. 
     
     
       3. An RMS detector circuit comprising: an input terminal;   an output terminal;   a squaring circuit for receiving a first signal and providing a squared signal having an amplitude which is proportional to the square of the amplitude of said first signal;   gain control means for automatically adjusting signals applied to said input signal terminal to a signal at a substantially constant level, said gain control means including feedback means for maintaining said substantially constant level an inverse function of signal amplitude at said output signal terminal;   means for applying said signal at substantially constant level to said squaring circuit as said first signal; and   low pass filter means for eliminating all but DC components in the squared output signal from said squaring circuit and applying said DC components to said output signal terminal;   wherein said squaring circuit comprises:   an operational transconductance amplifier having an input terminal, a bias control terminal and an output terminal which provides an output current proportional to the product of the voltage applied to said input terminal times the current applied to said bias control terminal;   means for applying said substantially constant level to said input terminal of said operational transconductance amplifier; and   means for applying a current proportional to said constant level to said bias control terminal.   
     
     
       4. The RMS detector according to claim 1 wherein said squaring circuit comprises: a second operational transconductance amplifier having an input terminal, a bias control terminal and an output terminal which provides an output current proportional to the product of the voltage applied to said input terminal times the current applied to said bias control terminal;   means for applying said substantially constant level to said input terminal of said second operational transconductance amplifier; and   means for applying a current proportional to said constant level to said bias control terminal of said second operational transconductance amplifier.   
     
     
       5. An RMS detector circuit for providing an output voltage proportional to the RMS value of an input voltage, said circuit comprising: an input port connected to receive said input voltage;   an output port for providing said output voltage;   squaring circuit means responsive to application of a first signal thereto for providing a second signal having an amplitude which is proportional to the square of the amplitude of said first signal;   gain-controlled amplifier means connected to receive said input voltage from said input port and provide said first signal;   gain control means for adjusting the gain of said gain-controlled amplifier to maintain the amplitude of said first signal substantially constant, said gain control means including feedback means connected between said output port and said gain-controlled amplifier for controlling the gain of said amplifier as an inverse function of said output voltage;   means applying said first signal to said squaring circuit;   low pass filter means connected to receive said second signal from said squaring circuit for removing all AC components from said second signal, said low pass filter means being further connected to pass all DC components of said second signal to said output port;   wherein said gain-controlled amplifier is a high gain inverting amplifier having an input terminal connected to receive said input voltage; and   wherein said gain control means comprises:   an operational transconductance amplifier having an input terminal connected to receive said first signal, a bias control terminal, and an output terminal connected to the input terminal of the high gain inverting amplifier, said transconductance amplifier being of the type which delivers an output current from its output terminal which is proportional to the product of the voltage at its input terminal times the current delivered to said bias control terminal; and   voltage to current converter means connected between said output port and said bias control terminal for delivering a bias current to said bias control terminal which is proportional to the output voltage at said output port.   
     
     
       6. The RMS detector circuit according to claim 5 wherein said squaring circuit comprises: a second operational transconductance amplifier having an input terminal for receiving said first signal, a bias control terminal and an output terminal which provides an output current proportional to the product of the voltage applied to said input terminal times the current applied to said bias control terminal; and   means for applying a current proportional to said constant level to said bias control terminal of said second operational transconductance amplifier.